The present invention relates to a helical slow-wave circuit assembly and, more particularly, to a helical slow-wave circuit assembly utilized in, e.g., a traveling-wave tube and a backward-wave tube.
Electron beams pass through a helical slow-wave circuit in a traveling-wave tube or a backward-wave tube as they partly come close to the circuit. Thus, the helical slow-wave circuit is heated by heat generated when the electron beams partly collide against the helical slow-wave circuit, or by heat generated by resistance loss of an RF power transmitted through the helical slow-wave circuit. Due to this heating function, a helical slow-wave circuit having a comparatively small heat capacity reaches a rather high temperature. This leads to an increase in RF power loss and an increase in gas emitted from the slow-wave circuit, leading to a decrease in output of the traveling- or backward-wave tube, degradation in beam transmittance, an increase in noise, and the like, as well as a decrease in service life. In recent years, demands for a higher frequency and a higher output of a traveling-wave tube are increasing. In a helical slow-wave tube used for these purposes, the dielectric constant and the heat conductivity of dielectric pillars are important factors in addition to the heat resistance of the helix and the means for emitting heat from the helix.
In a conventional helical slow-wave circuit assembly, a helix is formed by using a round wire or a tape, and a plurality of cylindrical or prismatic dielectric pillars are disposed around the helix. This structure is housed in a metal cylinder, and the helix and the dielectric pillars are clamped and fixed by using an appropriate means. An example of this assembly will be described with reference to FIGS. 4A and 4B.
FIG. 4A is a sectional view of a conventional helical slow-wave circuit assembly, and FIG. 4B is a partial enlarged sectional view of a portion in the vicinity of a boron nitride (to be described as P-BN hereinafter) pillar of this assembly. As shown in FIGS. 4A and 4B, a helix 1 (see FIG. 4A) constituting the slow-wave circuit is made by forming tungsten or molybdenum, which is not easily softened or deformed by collision of electron beams and which has a comparatively high melting point, into a wire or a tape, and coiling it in a helical manner. Three prismatic P-BN pillars 2, 3, 4 are disposed around the helix 1 at angular intervals of 120.degree., and a metal cylinder 11 is provided to surround the P-BN pillars 2, 3, 4. The P-BN pillars 2 to 4 have a multilayered structure.
A direction of each P-BN pillar parallel to the layers is called a direction a, and a direction thereof perpendicular to the layers is called a direction c. A cross in FIG. 4A represents the tail feathers of an arrow traveling in direction c down the traveling wave tube. Generally, in P-BN, the directions a and c have largely different physical and mechanical characteristics, and characteristics in the direction a are superior to those in the direction c. Therefore, the P-BN pillars 2, 3, 4 are provided such that the directions a and c become respectively perpendicular and parallel to the contact surfaces between the helix 1 with the P-BN pillars 2, 3, 4.
In order to prevent a concentration of the mechanical stress which occurs upon insertion of the P-BN pillars 2, 3, 4 into the metal cylinder 11, the outer and inner circumferential surfaces of the respective P-BN pillars 2, 3, 4 are formed in accordance with a radius R of curvature of the metal cylinder 11 and the helix 1. Furthermore, conventionally, artificial diamond films 5, 6, 7 having a thickness of several .mu.m are formed on the outer circumferential surfaces of the P-BN pillars 2, 3, 4, respectively, in accordance with chemical vapor deposition (to be referred to as CVD hereinafter), ion plating (to be referred to as IP hereinafter), or the like in order to increase the heat conductivity and mechanical strength.
The mechanical strength of the P-BN pillars 2, 3, 4 need be increased due to the following reason. More specifically, when the helix 1 and the P-BN pillars 2, 3, 4 are clamped by the metal cylinder 11, shearing stresses act on the helix 1 at portions corresponding to the central portions of the P-BN pillars and on the metal cylinder 11 corresponding to the two ends of each P-BN pillar. As described above, however, because of the characteristics and the manufacture of the P-BN pillars, the P-BN pillars 2, 3, 4, the helix 1, and the metal cylinder 11 contact with each other through the surfaces in the axis of the direction c.
When the helix 1 and the P-BN pillars 2, 3, 4 are clamped by the metal cylinder 11 in accordance with a squeezing to be described later, a shearing stress acts on the P-BN pillars 2, 3, 4, causing many cracks. This cracking adversely affects the RF characteristics of the traveling-wave tube and decreases the output and gain of the traveling-wave tube. Furthermore, when the cracks formed in the P-BN pillars 2, 3, 4 during clamping are subjected to thermal hysteresis and progress due to the operation of the traveling-wave tube, the traveling-wave tube may cause an operation error, which is a critical defect.
The artificial diamond films 5, 6, 7 are respectively formed on the outer circumferential surfaces of the P-BN pillars 2, 3, 4 in order to eliminate these drawbacks. Regarding the metal cylinder 11, since a means for applying a magnetic field for focusing the electron beams traveling in the helix 1 is to be arranged around the metal cylinder 11, mainly a stainless-steel tube, and recently, a tube constituted by layers of iron and a copper alloy and serving also as a vacuum envelope and enabled an overall reduction in size, are used as the metal cylinder 11.
To insert the helix 1 and the P-BN pillars 2, 3, 4 having the respective diamond films 5, 6, 7 into the metal cylinder 11, for example, the metal cylinder 11 is heated to utilize thermal expansion; or, a pressure is applied to the outer surface of the metal cylinder 11 in three directions to utilize mechanical deformation (squeezing). After insertion, heat or pressure is removed from the metal cylinder 11, so that the helix 1 and the P-BN pillars 2, 3, 4 are fixed and clamped, thereby completing a helical slow-wave circuit assembly.
In the conventional helical slow-wave circuit assembly described above, however, the P-BN pillars 2, 3, 4 are heated when the artificial diamond films 5, 6, 7 are respectively formed on the P-BN pillars 2, 3, 4 in accordance with CVD or IP processing. When the traveling- or backward-wave tube operates, nitrogen (N) in the P-BN pillars 2, 3, 4 are respectively diffused to the artificial diamond films 5, 6, 7, thereby decreasing the electrical resistance of the artificial diamond films 5, 6, 7. More specifically, diffusion of nitrogen (N) caused by heating the P-BN pillars 2, 3, 4 decreases the electrical resistance (resistivity) of the respective artificial diamond films 5, 6, 7 from 10.sup.11 .OMEGA..multidot.cm to as low as 10.sup.5 to 10.sup.6 .OMEGA..multidot.cm. When the electrical resistance of the surfaces of the P-BN pillars 2, 3, 4 serving as dielectric pillars is decreased in this manner, loss of the RF wave amplified while being transmitted through the helix 1 becomes considerably large, thereby extremely decreasing the output of the traveling- or backward-wave tube, which is a critical drawback.